when既可表示某一点时间,也可以表示某一段时间 .
在when引导的时间状语从句中,其谓语动词可以是延续性的,也可以是非延续性的,可与主句中的谓语动词同时发生,也可在其后发生.
一、延续性动词
延续性动词就是指能延续的动作,可以持续下去的动作,比如说read,它可以长时间延续,可以一直reading下去.如：live,run,stay,clean,play,hold,watch,teach,read,study,teach,eat,drink,write,dance,sing,smoke
二、when和while的区别与用法
1.when 是指某个时间所发生的事,用于在完成时态
while是指某个时间段所发生的事,用于进行时态
2.①when意为"在……时刻或时期",它可兼指"时间点"与"时间段",所引导的从句的动词既可以是终止性动词,也可是持续性动词.如：When I got home,he was having supper.我到家时,他正在吃饭.When I was young,I liked dancing.我年轻时喜欢跳舞.
②while只指"时间段",不指"时间点",从句的动词只限于持续性动词.如：While I slept,a thief broke in.在我睡觉时,盗贼闯了进来.
③when从句动词为终止性动词时,不能由while替换.如：When he came yesterday,we were playing basketball.昨天他来时,我们正在打篮球.
④当从句的谓语是表动作的延续性动词时,when,while才有可能互相替代.如：While / When we were still laughing,the teacher came in.正当我们仍在大声嬉笑时,老师进来了.
⑤当从句的谓语动词是终止性动词,而且主句的谓语动词也是终止性动词时,when可和as通用,而且用as比用when在时间上更为紧凑,有"正当这时"的含义.如：He came just as (or when) I reached the door.我刚到门那儿,他就来了.
⑥从句的谓语动词如表示状态时,通常用while.如：We must strike while the iron is hot.我们应该趁热打铁.
⑦while和when都可以用作并列连词.Mike is tall while his brother is short.迈克的个子很高,然而他弟弟很矮.Mary was on her way home when two big boys stopped her.玛丽正走在回家的路上,这时两个大个子男孩拦住她.
